There are at least two different heat stable (ST) toxins produced by E. coli. I have shown that the genetic information for the biosynthesis of at least one of these toxins lies on a plasmid and that the gene acts like a transposon much like many genes encoding antibiotic resistance. Using the technique of genetic engineering I hope to clone the gene for each of these toxins into as small a restriction enzyme fragment as possible which will still retain the ST activity. The nucleotide sequence of these cloned DNA fragments will be determined using the technique of sequencing DNA developed by Maxam and Gilbert. The nucleotide sequence of the ST gene will yield valuable information on: (1) the regulation and expression of the toxin genes and (2) the structure of the toxins themselves and how related they are to each other. Again using recombinant DNA technology these cloned DNA fragments containing the toxin genes will be ligated to fragments encoding the promoter operator region of the E. coli lac operon plus the partial sequence encoding the lacZ protein. When the expression of ST is under lac control, the synthesis of the toxins can be turned on and off at will using the classical inducers and repressors defined for the manipulation of Beta-galactosidase production. Having a portion of the lacA gene linked onto the toxin gene will hopefully result in a fusion peptide. The methanol soluble ST is nonimmunogenic, most probably because of its size (4500 mol. wt.). The ST-lacZ hybrid protein will be large enough (over 100,000 mol. wt.) to overcome the size threshold for antigenicity. This can serve as a means for obtaining ST vaccines if the antibodies elicited against the fusion peptide is also active in neutralizing the ST moiety of the hybrid. Using the cloned DNA fragments encoding the two types of ST as probes, I plan also to adapt existing techniques for DNA-DNA hybridization for use by clinical laboratories to detect rapidly E. coli ST producers in strains isolated from diarrheal outbreaks.